Using an interferometer as a high speed variable attenuator

ABSTRACT

A system and method provides high speed variable attenuators. The attenuators can be used within a lithographic apparatus to control intensity of radiation in one or more correction pulses used to correct a dose of the radiation following an initial pulse of radiation.

BACKGROUND

1. Field of the Invention

The present invention relates to a variable attenuator, a lithographicapparatus and a method for manufacturing a device.

2. Related Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate or part of a substrate. A lithographic apparatus can beused, for example, in the manufacture of flat panel displays, integratedcircuits (ICs) and other devices involving fine structures. In aconventional apparatus, a patterning device, which can be referred to asa mask or a reticle, can be used to generate a circuit patterncorresponding to an individual layer of a flat panel display (or otherdevice). This pattern can be transferred onto all or part of thesubstrate (e.g., a glass plate), by imaging onto a layer ofradiation-sensitive material (e.g., resist) provided on the substrate.

Instead of a circuit pattern, the patterning device can be used togenerate other patterns, for example a color filter pattern or a matrixof dots. Instead of a mask, the patterning device can be a patterningarray that comprises an array of individually controllable elements. Thepattern can be changed more quickly and for less cost in such a systemcompared to a mask-based system.

A flat panel display substrate is typically rectangular in shape.Lithographic apparatus designed to expose a substrate of this type canprovide an exposure region that covers a full width of the rectangularsubstrate, or covers a portion of the width (for example half of thewidth). The substrate can be scanned underneath the exposure region,while the mask or reticle is synchronously scanned through a beam. Inthis way, the pattern is transferred to the substrate. If the exposureregion covers the full width of the substrate then exposure can becompleted with a single scan. If the exposure region covers, forexample, half of the width of the substrate, then the substrate can bemoved transversely after the first scan, and a further scan is typicallyperformed to expose the remainder of the substrate.

The radiation sources typically used with a lithographic apparatusinclude pulsed laser sources. Typically, for mask-based lithographicapparatus, excimer lasers are used and several tens of laser pulses areused to expose each pattern on a part of a substrate. A problem withexcimer lasers is that there is a random variation of the pulse energyof plus or minus 10% of the intended energy for each pulse. However, byusing a fast control algorithm and the fact that the exposure dose onthe substrate is typically made up from 40 to 60 pulses, the variationof the energy dose received at the substrate is typically of the orderof plus or minus 0.1% or below.

In a maskless apparatus, the pattern set by the patterning device can beimaged onto the substrate using a single pulse of the radiation system.This is because the size of the image projected onto the substrates atany one instant is relatively small and in order to provide an adequatethroughput of substrate through the lithographic apparatus. However, fora single pulse, as discussed above, the energy variation can be plus orminus 10%. Such a variation in the energy of the pulse results in anunacceptably high variation in the line width produced on the substrate.

In order to provide the required radiation dose control, it has beenproposed for maskless lithographic systems to employ an arrangement inwhich the total dose of radiation is comprised of a main pulse and oneor more correction pulses. In such an arrangement, the main pulseprovides the large majority of the dose of radiation. The energy withinthe pulse of radiation is measured and it is subsequently determined howmuch additional radiation is required to provide the required dose. Acorrection pulse is then provided in which the radiation source is setto provide a second full pulse, but the pulse is passed through avariable attenuator that is set to reduce the energy within the pulse tothe required level.

For example, the main pulse can provide 90% of the dose required.Accordingly, for the correction pulse, the attenuator is set to reducethe energy in the correction pulse, such that it only transmits theenergy required to complete the dose of radiation. If the radiationsource generates pulses that nominally provide 90% of the total doserequired, the attenuator is set for the correction pulse to only passone ninth of the pulse of radiation, e.g., to provide the final 10% ofthe required dose.

Such an arrangement can allow for the energy within the main pulse to beaccurately measured. The attenuator can be set to pass accurately arequisite portion of the correction pulse. However, the potential errorin the dose provided by the correction pulse (which originates from thevariation in the energy in the pulses generated by the radiation source)is also reduced by the attenuator. Accordingly, the overall accuracy ofthe dose is improved.

A further improvement can be provided, for example, by utilizing a thirdcorrection pulse in which the intensity is further attenuated. Forexample, the main pulse can nominally provide 90% of the dose, the firstcorrection pulse can nominally provide 9% of the dose and the secondcorrection pulse can nominally provide 1% of the required dose. Such anarrangement can provide a dose accuracy that is one hundred times betterthan the dose accuracy of the radiation source.

However, for such an arrangement to be usable within a lithographicapparatus, the variable attenuator must meet high performance criteria.Firstly, the variable attenuator must be capable of being set veryaccurately to given levels of transmission. Secondly, the variableattenuator must be able to switch between different transmission levelsvery quickly (the time between successive pulses can be of the order of166 μs). Thirdly, the variable attenuator should be switchable between arelatively high level of transmission and a relatively low level oftransmission. Fourthly, the variable attenuator must be able to operatestably for as long a product lifetime as possible. Finally, the variableattenuator must be able to meet these performance criteria for radiationat the wavelength to be used in the lithographic process. A variableattenuator that meets these performance criteria is not presently known.

Therefore, what is needed is a system and method for providing avariable attenuator that meets the performance criteria necessary foruse in a subsystem of a lithographic apparatus used to control the doseof radiation in a lithographic exposure process.

SUMMARY

In one embodiment of the present invention, there is provided a variableattenuator configured to adjust its level of transmission to an inputbeam of radiation in response to an input control signal, whichrepresents a desired level of transmission of the variable attenuator tothe beam of radiation. The variable attenuator comprises first andsecond semi-transparent reflectors and an actuator systems. The firstand second semi-transparent reflectors are arranged substantiallymutually parallel, such that the beam of radiation successively passesthrough the first and second semi-transparent reflectors. The actuatorsystem is configured to control the separation of the first and secondsemi-transparent reflectors in response to the input control signal.

In another embodiment of the present invention, there is provided avariable attenuator configured to adjust its level of transmission to aninput beam of radiation in response to an input control signal, whichrepresents a desired level of transmission of the variable attenuator tothe beam of radiation. The variable attenuator comprises a radiationbeam splitter, a radiation beam combiner, radiation beam pathlengthcontroller. The radiation beam splitter divides the beam of radiationinto first and second radiation beam paths. The radiation beam combinerre-combines radiation from the first and second radiation beam paths,such that it interferes and produces an output beam of radiation. Theradiation beam pathlength controller is configured to control thepathlength of the first radiation beam path in response to the inputcontrol signal in order to control the interference of the radiationfrom the first and second radiation beam paths.

In an further embodiment of the present invention, there is provided avariable attenuator configured to adjust its level of transmission to aninput beam of radiation in response to an input control signal, whichrepresents a desired level of transmission of the variable attenuator tothe beam of radiation. The variable attenuator comprises first andsecond phase gratings and an actuator system. The first and second phasegratings are arranged substantially mutually parallel. The beam ofradiation is initially incident on the first phase grating, and afterpassing through the first phase grating, is incident on the second phasegrating. Each of the phase gratings comprises a plurality of regions ofa first type and a plurality of regions of a second type. The phasegratings are constructed such that, for each phase grating, the phaseshift introduced to radiation passing through the regions of the firsttype is a quarter of the wavelength of the beam of radiation input tothe variable attenuator greater or less than for the regions of thesecond type. The actuator system is configured to adjust the relativepositions of the first and second phase gratings in response to theinput control signal at least between a first position and a secondposition. In the first position, radiation passing through regions ofthe first and second type of the first phase grating subsequently passesthrough regions of the first and second type, respectively, of thesecond phase grating. In the second position, radiation passing throughregions of the first and second type of the first grating subsequentlypasses through regions of the second and first type, respectively, ofthe second phase grating.

In one embodiment there is provided a lithographic apparatusincorporating a variable attenuator such as discussed above.

In a yet further embodiment there is provided a device manufacturingmethod, comprising the following steps. Modulating a pulsed beam ofradiation and projecting it onto a substrate. The intensity of at leastone pulse of the pulsed beam of radiation is, prior to being modulated,attenuated by a variable attenuator configured to adjust its level oftransmission to an input beam of radiation in response to an inputcontrol signal, which represents a desired level of transmission of thevariable attenuator to the beam of radiation. The variable attenuatorcomprises first and second semi-transparent reflectors that are arrangedsubstantially mutually parallel. The beam of radiation successivelypasses through the first and second semi-transparent reflectors. Themethod further comprises controlling the separation of the first andsecond semi-transparent reflectors in response to the input controlsignal.

In a yet another embodiment, there is provided a device manufacturingmethod, comprising the following steps. Modulating a pulsed beam ofradiation and projecting it onto a substrate. The intensity of at leastone pulse of the pulsed beam of radiation is, prior to being modulated,attenuated by a variable attenuator configured to adjust its level oftransmission to an input beam of radiation in response to an inputcontrol signal which represents a desired level of transmission of thevariable attenuator to the beam of radiation. The variable attenuatorcomprises a radiation beam splitter that divides the beam of radiationinto first and second radiation beam paths and a radiation beam combinerthat re-combines radiation from the first and second radiation beampaths such that it interferes and produces an output beam of radiation.The method comprises using a radiation beam pathlength controller tocontrol the pathlength of the first radiation beam path in response tothe input control signal in order to control the interference of theradiation from the first and second radiation beam paths.

In a yet still further embodiment, there is provided a devicemanufacturing method comprising the following steps. Modulating a pulsedbeam of radiation and projecting it onto a substrate. The intensity ofat least one pulse of the pulsed beam of radiation is, prior to beingmodulated, attenuated by a variable attenuator configured to adjust itslevel of transmission to an input beam of radiation in response to aninput control signal, which represents a desired level of transmissionof the variable attenuator to the beam of radiation. The variableattenuator comprises first and second phase gratings, arrangedsubstantially mutually parallel, such that the beam of radiation isinitially incident on the first phase grating and, having passed throughthe first phase grating, is incident on the second phase grating. Eachof the phase gratings comprises a plurality of regions of a first typeand a plurality of regions of a second type. The phase gratings areconstructed such that, for each phase grating, the phase shiftintroduced to radiation passing through the regions of the first type isa quarter of the wavelength of the beam of radiation input to thevariable attenuator greater than for the regions of the second type. Themethod further comprises adjusting the relative positions of the firstand second phase gratings in response to the input control signal atleast between a first position and a second position. In the firstposition, radiation passing through regions of the first and second typeof the first phase rating subsequently passes through regions of thefirst and second type, respectively, of the second phase grating. In thesecond position, radiation passing through regions of the first andsecond type of the first grating subsequently passes through regions ofthe second and first type, respectively, of the second phase grating.

Further embodiments, features, and advantages of the present inventions,as well as the structure and operation of the various embodiments of thepresent invention, are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate one or more embodiments of the presentinvention and, together with the description, further serve to explainthe principles of the invention and to enable a person skilled in thepertinent art to make and use the invention.

FIGS. 1 and 2 depict lithographic apparatus, according to variousembodiments of the present invention.

FIG. 3 depicts a mode of transferring a pattern to a substrate accordingto one embodiment of the invention as shown in FIG. 2.

FIG. 4 depicts an arrangement of optical engines, according to oneembodiment of the present invention.

FIG. 5 depicts the use of a variable attenuator, according to thepresent invention within a lithographic apparatus.

FIGS. 6 and 7 depict a first embodiment of variable attenuator,according to the present invention.

FIG. 8 depicts a second embodiment of a variable attenuator, accordingto the present invention.

FIG. 9 a depicts a third embodiment of a variable attenuator, accordingto the present invention.

FIG. 9 b depicts a detail of a variable attenuator, according to thethird embodiment of the present invention.

FIG. 10 depicts a fourth embodiment of a variable attenuator, accordingto the present invention.

FIG. 11 depicts a fifth embodiment of the variable attenuator, accordingto the present invention.

FIGS. 12 a and 12 b depict an operating principle of an alternative kindof variable attenuator, according to the present invention within alithographic apparatus.

FIG. 12 c depicts phase gratings separated by a Talbot spacing.

FIG. 13 depicts a sixth embodiment of a variable, according to thepresent invention within a lithographic apparatus.

FIG. 14 depicts a seventh embodiment of a variable attenuator, accordingto the present invention within a lithographic apparatus.

FIG. 15 depicts an eighth embodiment of a variable, according to thepresent invention within a lithographic apparatus.

One or more embodiments of the present invention will now be describedwith reference to the accompanying drawings. In the drawings, likereference numbers can indicate identical or functionally similarelements. Additionally, the left-most digit(s) of a reference number canidentify the drawing in which the reference number first appears.

DETAILED DESCRIPTION

This specification discloses one or more embodiments that incorporatethe features of this invention. The disclosed embodiment(s) merelyexemplify the invention. The scope of the invention is not limited tothe disclosed embodiment(s). The invention is defined by the claimsappended hereto.

The embodiment(s) described, and references in the specification to “oneembodiment”, “an embodiment”, “an example embodiment”, etc., indicatethat the embodiment(s) described can include a particular feature,structure, or characteristic, but every embodiment may not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases are not necessarily referring to the same embodiment.Further, when a particular feature, structure, or characteristic isdescribed in connection with an embodiment, it is understood that it iswithin the knowledge of one skilled in the art to effect such feature,structure, or characteristic in connection with other embodimentswhether or not explicitly described.

Embodiments of the invention can be implemented in hardware, firmware,software, or any combination thereof. Embodiments of the invention canalso be implemented as instructions stored on a machine-readable medium,which can be read and executed by one or more processors. Amachine-readable medium can include any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputing device). For example, a machine-readable medium can includeread only memory (ROM); random access memory (RAM); magnetic diskstorage media; optical storage media; flash memory devices; electrical,optical, acoustical or other forms of propagated signals (e.g., carrierwaves, infrared signals, digital signals, etc.), and others. Further,firmware, software, routines, instructions can be described herein asperforming certain actions. However, it should be appreciated that suchdescriptions are merely for convenience and that such actions in factresult from computing devices, processors, controllers, or other devicesexecuting the firmware, software, routines, instructions, etc.

FIG. 1 schematically depicts the lithographic apparatus 1 of oneembodiment of the invention. The apparatus comprises an illuminationsystem IL, a patterning device PD, a substrate table WT, and aprojection system PS. The illumination system (illuminator) IL isconfigured to condition a radiation beam B (e.g., UV radiation).

It is to be appreciated that, although the description is directed tolithography, the patterned device PD can be formed in a display system(e.g., in a LCD television or projector), without departing from thescope of the present invention. Thus, the projected patterned beam canbe projected onto many different types of objects, e.g., substrates,display devices, etc.

The substrate table WT is constructed to support a substrate (e.g., aresist-coated substrate) W and connected to a positioner PW configuredto accurately position the substrate in accordance with certainparameters.

The projection system (e.g., a refractive projection lens system) PS isconfigured to project the beam of radiation modulated by the array ofindividually controllable elements onto a target portion C (e.g.,comprising one or more dies) of the substrate W. The term “projectionsystem” used herein should be broadly interpreted as encompassing anytype of projection system, including refractive, reflective,catadioptric, magnetic, electromagnetic and electrostatic opticalsystems, or any combination thereof, as appropriate for the exposureradiation being used, or for other factors such as the use of animmersion liquid or the use of a vacuum. Any use of the term “projectionlens” herein can be considered as synonymous with the more general term“projection system.”

The illumination system can include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The patterning device PD (e.g., a reticle or mask or an array ofindividually controllable elements) modulates the beam. In general, theposition of the array of individually controllable elements will befixed relative to the projection system PS. However, it can instead beconnected to a positioner configured to accurately position the array ofindividually controllable elements in accordance with certainparameters.

The term “patterning device” or “contrast device” used herein should bebroadly interpreted as referring to any device that can be used tomodulate the cross-section of a radiation beam, such as to create apattern in a target portion of the substrate. The devices can be eitherstatic patterning devices (e.g., masks or reticles) or dynamic (e.g.,arrays of programmable elements) patterning devices. For brevity, mostof the description will be in terms of a dynamic patterning device,however it is to be appreciated that a static pattern device can also beused without departing from the scope of the present invention.

It should be noted that the pattern imparted to the radiation beam maynot exactly correspond to the desired pattern in the target portion ofthe substrate, for example if the pattern includes phase-shiftingfeatures or so called assist features. Similarly, the pattern eventuallygenerated on the substrate may not correspond to the pattern formed atany one instant on the array of individually controllable elements. Thiscan be the case in an arrangement in which the eventual pattern formedon each part of the substrate is built up over a given period of time ora given number of exposures during which the pattern on the array ofindividually controllable elements and/or the relative position of thesubstrate changes.

Generally, the pattern created on the target portion of the substratewill correspond to a particular functional layer in a device beingcreated in the target portion, such as an integrated circuit or a flatpanel display (e.g., a color filter layer in a flat panel display or athin film transistor layer in a flat panel display). Examples of suchpatterning devices include reticles, programmable mirror arrays, laserdiode arrays, light emitting diode arrays, grating light valves, and LCDarrays.

Patterning devices whose pattern is programmable with the aid ofelectronic means (e.g., a computer), such as patterning devicescomprising a plurality of programmable elements (e.g., all the devicesmentioned in the previous sentence except for the reticle), arecollectively referred to herein as “contrast devices.” The patterningdevice comprises at least 10, at least 100, at least 1,000, at least10,000, at least 100,000, at least 1,000,000, or at least 10,000,000programmable elements.

A programmable mirror array can comprise a matrix-addressable surfacehaving a viscoelastic control layer and a reflective surface. The basicprinciple behind such an apparatus is that addressed areas of thereflective surface reflect incident light as diffracted light, whereasunaddressed areas reflect incident light as undiffracted light. Using anappropriate spatial filter, the undiffracted light can be filtered outof the reflected beam, leaving only the diffracted light to reach thesubstrate. In this manner, the beam becomes patterned according to theaddressing pattern of the matrix-addressable surface.

It will be appreciated that, as an alternative, the filter can filterout the diffracted light, leaving the undiffracted light to reach thesubstrate.

An array of diffractive optical MEMS devices (micro-electro-mechanicalsystem devices) can also be used in a corresponding manner. In oneexample, a diffractive optical MEMS device is composed of a plurality ofreflective ribbons that can be deformed relative to one another to forma grating that reflects incident light as diffracted light.

A further alternative example of a programmable mirror array employs amatrix arrangement of tiny mirrors, each of which can be individuallytilted about an axis by applying a suitable localized electric field, orby employing piezoelectric actuation means. Once again, the mirrors arematrix-addressable, such that addressed mirrors reflect an incomingradiation beam in a different direction than unaddressed mirrors; inthis manner, the reflected beam can be patterned according to theaddressing pattern of the matrix-addressable mirrors. The requiredmatrix addressing can be performed using suitable electronic means.

Another example PD is a programmable LCD array.

The lithographic apparatus can comprise one or more contrast devices.For example, it can have a plurality of arrays of individuallycontrollable elements, each controlled independently of each other. Insuch an arrangement, some or all of the arrays of individuallycontrollable elements can have at least one of a common illuminationsystem (or part of an illumination system), a common support structurefor the arrays of individually controllable elements, and/or a commonprojection system (or part of the projection system).

In one example, such as the embodiment depicted in FIG. 1, the substrateW has a substantially circular shape, optionally with a notch and/or aflattened edge along part of its perimeter. In another example, thesubstrate has a polygonal shape, e.g., a rectangular shape.

Examples where the substrate has a substantially circular shape includeexamples where the substrate has a diameter of at least 25 mm, at least50 mm, at least 75 mm, at least 100 mm, at least 125 mm, at least 150mm, at least 175 mm, at least 200 mm, at least 250 mm, or at least 300mm. Alternatively, the substrate has a diameter of at most 500 mm, atmost 400 mm, at most 350 mm, at most 300 mm, at most 250 mm, at most 200mm, at most 150 mm, at most 100 mm, or at most 75 mm.

Examples where the substrate is polygonal, e.g., rectangular, includeexamples where at least one side, at least 2 sides or at least 3 sides,of the substrate has a length of at least 5 cm, at least 25 cm, at least50 cm, at least 100 cm, at least 150 cm, at least 200 cm, or at least250 cm.

At least one side of the substrate has a length of at most 1000 cm, atmost 750 cm, at most 500 cm, at most 350 cm, at most 250 cm, at most 150cm, or at most 75 cm.

In one example, the substrate W is a wafer, for instance a semiconductorwafer. The wafer material can be selected from the group consisting ofSi, SiGe, SiGeC, SIC, Ge, GaAs, InP, and InAs. The wafer can be: a III/Vcompound semiconductor wafer, a silicon wafer, a ceramic substrate, aglass substrate, or a plastic substrate. The substrate can betransparent (for the naked human eye), colored, or absent a color.

The thickness of the substrate can vary and, to an extent, can depend onthe substrate material and/or the substrate dimensions. The thicknesscan be at least 50 μm, at least 100 μm, at least 200 μm, at least 300μm, at least 400 μm, at least 500 μm, or at least 600 μm. Alternatively,the thickness of the substrate can be at most 5000 μm, at most 3500 μm,at most 2500 μm, at most 1750 μm, at most 1250 μm, at most 1000 μm, atmost 800 μm, at most 600 μm, at most 500 μm, at most 400 μm, or at most300 μm.

The substrate referred to herein can be processed, before or afterexposure, in for example a track (a tool that typically applies a layerof resist to a substrate and develops the exposed resist), a metrologytool, and/or an inspection tool. In one example, a resist layer isprovided on the substrate.

The projection system can image the pattern on the array of individuallycontrollable elements, such that the pattern is coherently formed on thesubstrate. Alternatively, the projection system can image secondarysources for which the elements of the array of individually controllableelements act as shutters. In this respect, the projection system cancomprise an array of focusing elements such as a micro lens array (knownas an MLA) or a Fresnel lens array to form the secondary sources and toimage spots onto the substrate. The array of focusing elements (e.g.,MLA) comprises at least 10 focus elements, at least 100 focus elements,at least 1,000 focus elements, at least 10,000 focus elements, at least100,000 focus elements, or at least 1,000,000 focus elements.

The number of individually controllable elements in the patterningdevice is equal to or greater than the number of focusing elements inthe array of focusing elements. One or more (e.g., 1,000 or more, themajority, or each) of the focusing elements in the array of focusingelements can be optically associated with one or more of theindividually controllable elements in the array of individuallycontrollable elements, with 2 or more, 3 or more, 5 or more, 10 or more,20 or more, 25 or more, 35 or more, or 50 or more of the individuallycontrollable elements in the array of individually controllableelements.

The MLA can be movable (e.g., with the use of one or more actuators) atleast in the direction to and away from the substrate. Being able tomove the MLA to and away from the substrate allows, e.g., for focusadjustment without having to move the substrate.

As herein depicted in FIGS. 1 and 2, the apparatus is of a reflectivetype (e.g., employing a reflective array of individually controllableelements). Alternatively, the apparatus can be of a transmission type(e.g., employing a transmission array of individually controllableelements).

The lithographic apparatus can be of a type having two (dual stage) ormore substrate tables. In such “multiple stage” machines, the additionaltables can be used in parallel, or preparatory steps can be carried outon one or more tables while one or more other tables are being used forexposure.

The lithographic apparatus can also be of a type wherein at least aportion of the substrate can be covered by an “immersion liquid” havinga relatively high refractive index, e.g., water, so as to fill a spacebetween the projection system and the substrate. An immersion liquid canalso be applied to other spaces in the lithographic apparatus, forexample, between the patterning device and the projection system.Immersion techniques are well known in the art for increasing thenumerical aperture of projection systems. The term “immersion” as usedherein does not mean that a structure, such as a substrate, must besubmerged in liquid, but rather only means that liquid is locatedbetween the projection system and the substrate during exposure.

Referring again to FIG. 1, the illuminator IL receives a radiation beamfrom a radiation source SO. The radiation source provides radiationhaving a wavelength of at least 5 nm, at least 10 nm, at least 11-13 nm,at least 50 nm, at least 100 nm, at least 150 nm, at least 175 nm, atleast 200 nm, at least 250 nm, at least 275 nm, at least 300 nm, atleast 325 nm, at least 350 nm, or at least 360 nm. Alternatively, theradiation provided by radiation source SO has a wavelength of at most450 nm, at most 425 nm, at most 375 nm, at most 360 nm, at most 325 nm,at most 275 nm, at most 250 nm, at most 225 nm, at most 200 nm, or atmost 175 nm. The radiation can have a wavelength including 436 nm, 405nm, 365 nm, 355 nm, 248 nm, 193 nm, 157 nm, and/or 126 nm.

The source and the lithographic apparatus can be separate entities, forexample when the source is an excimer laser. In such cases, the sourceis not considered to form part of the lithographic apparatus and theradiation beam is passed from the source SO to the illuminator IL withthe aid of a beam delivery system BD comprising, for example, suitabledirecting mirrors and/or a beam expander. In other cases the source canbe an integral part of the lithographic apparatus, for example when thesource is a mercury lamp. The source SO and the illuminator IL, togetherwith the beam delivery system BD if required, can be referred to as aradiation system.

The illuminator IL, can comprise an adjuster AD for adjusting theangular intensity distribution of the radiation beam. Generally, atleast the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL can comprise various other components, such as anintegrator IN and a condenser CO. The illuminator can be used tocondition the radiation beam to have a desired uniformity and intensitydistribution in its cross-section. The illuminator IL, or an additionalcomponent associated with it, can also be arranged to divide theradiation beam into a plurality of sub-beams that can, for example, eachbe associated with one or a plurality of the individually controllableelements of the array of individually controllable elements. Atwo-dimensional diffraction grating can, for example, be used to dividethe radiation beam into sub-beams. In the present description, the terms“beam of radiation” and “radiation beam” encompass, but are not limitedto, the situation in which the beam is comprised of a plurality of suchsub-beams of radiation.

The radiation beam B is incident on the patterning device PD (e.g., anarray of individually controllable elements) and is modulated by thepatterning device. Having been reflected by the patterning device PD,the radiation beam B passes through the projection system PS, whichfocuses the beam onto a target portion C of the substrate W. With theaid of the positioner PW and position sensor IF2 (e.g., aninterferometric device, linear encoder, capacitive sensor, or the like),the substrate table WT can be moved accurately, e.g., so as to positiondifferent target portions C in the path of the radiation beam B. Whereused, the positioning means for the array of individually controllableelements can be used to correct accurately the position of thepatterning device PD with respect to the path of the beam B, e.g.,during a scan.

In one example, movement of the substrate table WT is realized with theaid of a long-stroke module (course positioning) and a short-strokemodule (fine positioning), which are not explicitly depicted in FIG. 1.In another example, a short stroke stage may not be present. A similarsystem can also be used to position the array of individuallycontrollable elements. It will be appreciated that the beam B canalternatively/additionally be moveable, while the object table and/orthe array of individually controllable elements can have a fixedposition to provide the required relative movement. Such an arrangementcan assist in limiting the size of the apparatus. As a furtheralternative, which can, e.g., be applicable in the manufacture of flatpanel displays, the position of the substrate table WT and theprojection system PS can be fixed and the substrate W can be arranged tobe moved relative to the substrate table WT. For example, the substratetable WT can be provided with a system for scanning the substrate Wacross it at a substantially constant velocity.

As shown in FIG. 1, the beam of radiation B can be directed to thepatterning device PD by means of a beam splitter BS configured such thatthe radiation is initially reflected by the beam splitter and directedto the patterning device PD. It should be realized that the beam ofradiation B can also be directed at the patterning device without theuse of a beam splitter. The beam of radiation can be directed at thepatterning device at an angle between 0 and 90°, between 5 and 85°,between 15 and 75°, between 25 and 65°, or between 35 and 55° (theembodiment shown in FIG. 1 is at a 90° angle). The patterning device PDmodulates the beam of radiation B and reflects it back to the beamsplitter BS which transmits the modulated beam to the projection systemPS. It will be appreciated, however, that alternative arrangements canbe used to direct the beam of radiation B to the patterning device PDand subsequently to the projection system PS. In particular, anarrangement such as is shown in FIG. 1 may not be required if atransmission patterning device is used.

The depicted apparatus can be used in several modes:

1. In step mode, the array of individually controllable elements and thesubstrate are kept essentially stationary, while an entire patternimparted to the radiation beam is projected onto a target portion C atone go (i.e., a single static exposure). The substrate table WT is thenshifted in the X and/or Y direction so that a different target portion Ccan be exposed. In step mode, the maximum size of the exposure fieldlimits the size of the target portion C imaged in a single staticexposure.

2. In scan mode, the array of individually controllable elements and thesubstrate are scanned synchronously while a pattern imparted to theradiation beam is projected onto a target portion C (i.e., a singledynamic exposure). The velocity and direction of the substrate relativeto the array of individually controllable elements can be determined bythe (de-) magnification and image reversal characteristics of theprojection system PS. In scan mode, the maximum size of the exposurefield limits the width (in the non-scanning direction) of the targetportion in a single dynamic exposure, whereas the length of the scanningmotion determines the height (in the scanning direction) of the targetportion.

3. In pulse mode, the array of individually controllable elements iskept essentially stationary and the entire pattern is projected onto atarget portion C of the substrate W using a pulsed radiation source. Thesubstrate table WT is moved with an essentially constant speed such thatthe beam B is caused to scan a line across the substrate W. The patternon the array of individually controllable elements is updated asrequired between pulses of the radiation system and the pulses are timedsuch that successive target portions C are exposed at the requiredlocations on the substrate W. Consequently, the beam B can scan acrossthe substrate W to expose the complete pattern for a strip of thesubstrate. The process is repeated until the complete substrate W hasbeen exposed line by line.

4. Continuous scan mode is essentially the same as pulse mode exceptthat the substrate W is scanned relative to the modulated beam ofradiation B at a substantially constant speed and the pattern on thearray of individually controllable elements is updated as the beam Bscans across the substrate W and exposes it. A substantially constantradiation source or a pulsed radiation source, synchronized to theupdating of the pattern on the array of individually controllableelements, can be used.

5. In pixel grid imaging mode, which can be performed using thelithographic apparatus of FIG. 2, the pattern formed on substrate W isrealized by subsequent exposure of spots formed by a spot generator thatare directed onto patterning device PD. The exposed spots havesubstantially the same shape. On substrate W the spots are printed insubstantially a grid. In one example, the spot size is larger than apitch of a printed pixel grid, but much smaller than the exposure spotgrid. By varying intensity of the spots printed, a pattern is realized.In between the exposure flashes the intensity distribution over thespots is varied.

Combinations and/or variations on the above described modes of use orentirely different modes of use can also be employed.

In lithography, a pattern is exposed on a layer of resist on thesubstrate. The resist is then developed. Subsequently, additionalprocessing steps are performed on the substrate. The effect of thesesubsequent processing steps on each portion of the substrate depends onthe exposure of the resist. In particular, the processes are tuned suchthat portions of the substrate that receive a radiation dose above agiven dose threshold respond differently to portions of the substratethat receive a radiation dose below the dose threshold. For example, inan etching process, areas of the substrate that receive a radiation doseabove the threshold are protected from etching by a layer of developedresist. However, in the post-exposure development, the portions of theresist that receive a radiation dose below the threshold are removed andtherefore those areas are not protected from etching. Accordingly, adesired pattern can be etched. In particular, the individuallycontrollable elements in the patterning device are set such that theradiation that is transmitted to an area on the substrate within apattern feature is at a sufficiently high intensity that the areareceives a dose of radiation above the dose threshold during theexposure. The remaining areas on the substrate receive a radiation dosebelow the dose threshold by setting the corresponding individuallycontrollable elements to provide a zero or significantly lower radiationintensity.

In practice, the radiation dose at the edges of a pattern feature doesnot abruptly change from a given maximum dose to zero dose even if theindividually controllable elements are set to provide the maximumradiation intensity on one side of the feature boundary and the minimumradiation intensity on the other side. Instead, due to diffractiveeffects, the level of the radiation dose drops off across a transitionzone. The position of the boundary of the pattern feature ultimatelyformed by the developed resist is determined by the position at whichthe received dose drops below the radiation dose threshold. The profileof the drop-off of radiation dose across the transition zone, and hencethe precise position of the pattern feature boundary, can be controlledmore precisely by setting the individually controllable elements thatprovide radiation to points on the substrate that are on or near thepattern feature boundary. These can be not only to maximum or minimumintensity levels, but also to intensity levels between the maximum andminimum intensity levels. This is commonly referred to as “grayscaling.”

Grayscaling provides greater control of the position of the patternfeature boundaries than is possible in a lithography system in which theradiation intensity provided to the substrate by a given individuallycontrollable element can only be set to two values (e.g., just a maximumvalue and a minimum value). At least 3, at least 4 radiation intensityvalues, at least 8 radiation intensity values, at least 16 radiationintensity values, at least 32 radiation intensity values, at least 64radiation intensity values, at least 128 radiation intensity values, orat least 256 different radiation intensity values can be projected ontothe substrate.

It should be appreciated that grayscaling can be used for additional oralternative purposes to that described above. For example, theprocessing of the substrate after the exposure can be tuned, such thatthere are more than two potential responses of regions of the substrate,dependent on received radiation dose level. For example, a portion ofthe substrate receiving a radiation dose below a first thresholdresponds in a first manner; a portion of the substrate receiving aradiation dose above the first threshold but below a second thresholdresponds in a second manner; and a portion of the substrate receiving aradiation dose above the second threshold responds in a third manner.Accordingly, grayscaling can be used to provide a radiation dose profileacross the substrate having more than two desired dose levels. Theradiation dose profile can have at least 2 desired dose levels, at least3 desired radiation dose levels, at least 4 desired radiation doselevels, at least 6 desired radiation dose levels or at least 8 desiredradiation dose levels.

It should further be appreciated that the radiation dose profile can becontrolled by methods other than by merely controlling the intensity ofthe radiation received at each point on the substrate, as describedabove. For example, the radiation dose received by each point on thesubstrate can alternatively or additionally be controlled by controllingthe duration of the exposure of the point. As a further example, eachpoint on the substrate can potentially receive radiation in a pluralityof successive exposures. The radiation dose received by each point can,therefore, be alternatively or additionally controlled by exposing thepoint using a selected subset of the plurality of successive exposures.

FIG. 2 depicts an arrangement of the apparatus according to the presentinvention that can be used, e.g., in the manufacture of flat paneldisplays. Components corresponding to those shown in FIG. 1 are depictedwith the same reference numerals. Also, the above descriptions of thevarious embodiments, e.g., the various configurations of the substrate,the contrast device, the MLA, the beam of radiation, etc., remainapplicable.

As shown in FIG. 2, the projection system PS includes a beam expander,which comprises two lenses L1, L2. The first lens L1 is arranged toreceive the modulated radiation beam B and focus it through an aperturein an aperture stop AS. A further lens AL can be located in theaperture. The radiation beam B then diverges and is focused by thesecond lens L2 (e.g., a field lens).

The projection system PS further comprises an array of lenses MLAarranged to receive the expanded modulated radiation B. Differentportions of the modulated radiation beam B, corresponding to one or moreof the individually controllable elements in the patterning device PD,pass through respective different lenses ML in the array of lenses MLA.Each lens focuses the respective portion of the modulated radiation beamB to a point which lies on the substrate W. In this way an array ofradiation spots S is exposed onto the substrate W. It will beappreciated that, although only eight lenses of the illustrated array oflenses 14 are shown, the array of lenses can comprise many thousands oflenses (the same is true of the array of individually controllableelements used as the patterning device PD).

FIG. 3 illustrates schematically how a pattern on a substrate W isgenerated using the system of FIG. 2, according to one embodiment of thepresent invention. The filled in circles represent the array of spots Sprojected onto the substrate W by the array of lenses MLA in theprojection system PS. The substrate W is moved relative to theprojection system PS in the Y direction as a series of exposures areexposed on the substrate W. The open circles represent spot exposures SEthat have previously been exposed on the substrate W. As shown, eachspot projected onto the substrate by the array of lenses within theprojection system PS exposes a row R of spot exposures on the substrateW. The complete pattern for the substrate is generated by the sum of allthe rows R of spot exposures SE exposed by each of the spots S. Such anarrangement is commonly referred to as “pixel grid imaging,” discussedabove.

It can be seen that the array of radiation spots S is arranged at anangle θ relative to the substrate W (the edges of the substrate lieparallel to the X and Y directions). This is done so that when thesubstrate is moved in the scanning direction (the Y-direction), eachradiation spot will pass over a different area of the substrate, therebyallowing the entire substrate to be covered by the array of radiationspots 15. The angle θ can be at most 20°, at most 10°, at most 5°, atmost 3°, at most 1°, at most 0.5°, at most 0.25°, at most 0.10°, at most0.05°, or at most 0.01°. Alternatively, the angle θ is at least 0.001°.

FIG. 4 shows schematically how an entire flat panel display substrate Wcan be exposed in a single scan using a plurality of optical engines,according to one embodiment of the present invention. In the exampleshown eight arrays SA of radiation spots S are produced by eight opticalengines (not shown), arranged in two rows R1, R2 in a “chess board”configuration, such that the edge of one array of radiation spots (e.g.,spots S in FIG. 3) slightly overlaps (in the scanning direction Y) withthe edge of the adjacent array of radiation spots. In one example, theoptical engines are arranged in at least 3 rows, for instance 4 rows or5 rows. In this way, a band of radiation extends across the width of thesubstrate W, allowing exposure of the entire substrate to be performedin a single scan. It will be appreciated that any suitable number ofoptical engines can be used. In one example, the number of opticalengines is at least 1, at least 2, at least 4, at least 8, at least 10,at least 12, at least 14, or at least 17. Alternatively, the number ofoptical engines is less than 40, less than 30 or less than 20.

Each optical engine can comprise a separate illumination system IL,patterning device PD and projection system PS as described above. It isto be appreciated, however, that two or more optical engines can shareat least a part of one or more of the illumination system, patterningdevice and projection system.

FIG. 5 depicts an arrangement in which a variable attenuator can be usedas part of a radiation dose control system within a lithographicapparatus. As shown, a beam of radiation 10, is provided by, forexample, an illumination system 11. The beam of radiation passes througha partial reflector 12, which permits the majority of the beam ofradiation to pass to the variable attenuator 13, but which diverts aportion of the beam of radiation to a radiation detector 14. Asdiscussed above, the illumination system provides a pulsed beam ofradiation 10 and the detector 14 is configured to determine the energywithin each pulse of radiation. It will be appreciated that the detector14 will be calibrated, such that from the portion of the pulsed beam ofradiation 10 that is diverted to the detector 14, it can determine theenergy within each pulse that is transmitted to the variable attenuator13. A dose controller 15 is provided that provides a control signal tothe variable attenuator 13 to set the level of transmission of thevariable attenuator 13 to the beam of radiation that is input to it. Thebeam of radiation 16, output from the variable attenuator 13 andattenuated by it in accordance with the control signal from the dosecontroller 15, subsequently passes to the remainder of the lithographicapparatus and can, for example, be modulated by an array of individuallycontrollable elements 17.

As discussed above, the radiation dose control system can be arrangedsuch that a first pulse of radiation is transmitted through the systemlargely unattenuated and provides the large majority of the radiationdose required. The radiation detector 14 determines the energy withinthat first pulse and the dose controller 15 determines the energyrequired in the subsequent correction pulse. The controller 15 sets thevariable attenuator 13 such that its level of transmission is at therequisite level to attenuate the subsequent pulse of radiation to therequired level. If required, this process can be repeated for additionalcorrection pulses.

It should be appreciated that variations of the arrangement depicted inFIG. 5 can be used. For example, the dose control system can be providedprior to the illumination system, e.g., between the radiation source andthe illumination system that conditions the beam of radiation. Inaddition, the radiation detector 14 and the partial reflector 12 can bearranged to determine the energy within a pulse of radiation after ithas been attenuated by the variable attenuator 13 (e.g., the partialreflector can be arranged to divert a portion of the beam of radiation16 that is output from the variable attenuator to the radiation detector14). Such an arrangement may be better able to compensate for any errorsin the control of the variable attenuator 13. It will further beappreciated that the radiation detector 14 can be a simple photodiodeand the calibration functions and an integration function necessary todetermine the total energy within a pulse of radiation (rather thansimply the intensity of the radiation at a given time within the pulse)can be provided by the dose controller 15. It should further beappreciated that the variable attenuator 13 of the present invention canbe used for applications other than the one illustrated in FIG. 5.

Embodiment 1

FIG. 6 depicts a variable attenuator 20 according to the firstembodiment of the present invention for controlling the intensity of abeam of radiation 21. The variable attenuator includes a pair of partialreflectors 22, 23. The partial reflectors 22, 23 are arranged parallelto each other and are set at a distance apart from each other. Theseparation is controlled by a system of actuators 24 that are controlledby a controller 25.

Interference of the radiation reflected between the partial reflectors22, 23 affects the level of transmission T of the variable attenuator.If:

the spacing between the surfaces of the partial reflectors 22, 23 is L

the reflection coefficient of the partial reflectors 22, 23 is R

the wavelength of the radiation is λ

and the angle of incidence of the light on the mirror is θ

then

${T = \frac{\left( {1 - R} \right)^{2}}{1 + R^{2} - {2R\;{\cos(\delta)}}}},{{{with}\mspace{14mu}\delta} = {\left( \frac{2\pi}{\lambda} \right)2L\;{\cos(\theta)}}}$

Accordingly, by relative movement of the two partial reflectors 22, 23of an order of magnitude of the wavelength of the radiation, the levelof transmission of the variable attenuator can be switched between amaximum and a minimum and to any value in between. Therefore, theactuator system can be comprised of, for example, one or morepiezoelectric actuators that are capable of adjusting the relativeposition of the first and second partial reflectors 22, 23 to a highaccuracy over the required range of movement very quickly.

In general, the range of values of the level of transmission that can begenerated by the variable attenuator is determined by the reflectioncoefficient of the partial reflectors R. For high values of R, thetransmission-separation curve becomes very sharp, enabling largerchanges in the level of transmission T for a smaller relative movementof the partial reflectors 22, 23, but equally increasing the sensitivityof the level of transmission T to position errors of the mirrors. Highervalues of the coefficient of the mirrors R also results in a variableattenuator that can attain lower minimum levels of transmission T. Thisis important because the lower the minimum level of transmissionattainable by the variable attenuator, the greater the final accuracy ofthe total dose of radiation provided.

FIG. 7 shows a variation of the arrangement depicted in FIG. 6. In thiscase, the partial reflectors are arranged such that the beam ofradiation is incident on the partial reflectors at a small angle θ. Anadvantage of directing the beam of radiation onto the partial reflectorsat a small angle is that radiation reflected by the partial reflectorsis not returned to the radiation source. The angle of incidence θ isselected to be sufficient to prevent such back reflections or reducethem to an allowable level. However, as the angle of incidence θincreases, the sensitivity of the level of transmission of the variableattenuator to variations in the relative angle of the two partialreflectors (which should ideally be perfectly parallel) increases. Inaddition, the attainable contrast of the variable attenuator can bereduced because the beam of radiation does not precisely interfere withitself but with a slightly shifted copy. Both of these effects can bereduced by minimizing the angle of incidence θ and by minimizing theseparation L.

Accurate control of the relative position of the two partial reflectorsis essential to provide accurate control of the level of transmission ofthe variable attenuator. Accordingly, the controller 25 can include amemory 26 that stores calibration data, for example relating to theachieved level of transmission of the variable attenuator to the controlsignals provided to the actuator system 24.

Embodiment 2

FIG. 8 depicts an arrangement of The variable attenuator of a secondembodiment of the present invention. The second embodiment correspondsto the first embodiment as depicted in FIGS. 6 and 7, but differs in thecontrol of the actuator system 34 used to control the relative positionof the partial reflectors 32, 33. In this case, position sensors 36 areused to measure the relative position of the two partial reflectors 32,33. The position sensors can, for example, be capacitive sensors. Theposition sensor 36 can be arranged simply to provide a measurement ofthe separation L between the surfaces of the partial reflectors 32, 33.However, by provision of two or more position sensors, it also becomespossible to measure the relative angle between the surfaces.Accordingly, the controller 35 may not only adjust the separation L ofthe partial reflectors 32, 33 in order to control the transmission levelof the variable attenuator, but can also control the actuator 34 inorder to adjust the relative angle between the partial reflectors 32,33, improving, for example, the uniformity of the intensity of theradiation across the output beam of radiation 37.

The controller 35 can be provided with a feedback system such that, forexample, it determines a desired relative position of the partialreflectors 32, 33 in order to provide a given level of transmission ofthe variable attenuator, determines a difference between the desiredrelative position and a measured relative position provided by theposition sensors 36, and provides the control signals to the actuatorsystem 34 necessary to reduce the difference. Such an arrangement canrequire less calibration in order to provide accurate transmissioncontrol despite mechanical and thermal drifts of the components andhysteresis and creep of the piezoelectric actuator system.

Embodiment 3

FIG. 9 a depicts a third embodiment of the present invention. Again, thevariable attenuator 40 of the third embodiment operates on the sameprinciple as the first and second embodiments, but differs in thecontrol of the relative position of the two partial reflectors 42, 43.In this case, a second beam of radiation 46 is passed through thepartial reflectors 42, 43 at a different angle of incidence to that ofthe beam of radiation 41 that is to be controlled. The second beam ofradiation 46 is also attenuated by the variable attenuator, andsubsequently inspected by a radiation detector 47. The second beam ofradiation 46 can be provided by a source 48 that generates a beam ofradiation at a different wavelength from the wavelength of the beam ofradiation 41 to be controlled. Likewise, it need not provide the sameintensity, and for practical reasons probably would not. Accordingly,because the requirements for the radiation source 48 for the second beamof radiation 46 are relatively low, it can be mounted within thevariable attenuator. Alternatively, the radiation source 48 can bemounted external to the variable attenuator 40, and can even be externalto a lithographic apparatus within which the variable attenuator isused. In any case, the radiation detector 47 is configured to detect theintensity of the radiation of the second beam of radiation 46 after ithas been attenuated by the partial reflectors 42, 43 of the variableattenuator. Accordingly, the controller 45 can directly determine thelevel of transmission of the variable attenuator to the second beam ofradiation 46.

It should be appreciated that if the accuracy of the control of theintensity of the second beam of radiation is not sufficiently high, asecond radiation detector can be provided in association with a partialreflector prior to the second beam of radiation 46 passing through thepartial reflectors 42, 43. This can be done so that a comparison can bemade of measured intensity values before and after the second beam ofradiation is attenuated by the variable attenuator. The controller 45can subsequently determine, from the level of transmission of thevariable attenuator to the second beam of radiation 46, the level oftransmission of the variable attenuator to the beam of radiation 41 tobe controlled. This determination can be based on a calculation usingthe known wavelengths of the two beams of radiation and the angles ofincidence of each on the partial reflectors 42, 43 or can be based oncalibration data.

As shown in FIG. 9 b, the detector unit can comprise a partial reflector49 that divides the second beam of radiation 46 to a first path thatleads to a photodiode 50, for example, that measures the intensity ofthe radiation, and a CCD 51, for example, that measures the uniformityof the intensity of the radiation across the second beam of radiation46. Accordingly, the photodiode 50 can quickly determine the intensityof the radiation, from which the controller 45 can determine the overalltransmission level of the variable attenuator and, accordingly, can sendcontrol signals to the actuator system 44 to adjust the separation ofthe partial reflectors 42, 43, as necessary, in order to provide arequired level of transmission. In addition, the CCD 51 can, moreslowly, provide measurements of the uniformity of the radiation acrossthe second beam of radiation 46 from which the controller 45 candetermine corrections that should be made to adjust the relative angleof the partial reflectors 42, 43 in order to maintain the uniformity ofthe radiation distribution of the beam of radiation 41 being controlledby the variable attenuator.

Embodiment 4

FIG. 10 depicts a variable attenuator 60 according to a fourthembodiment of the present invention. In this embodiment, the beam ofradiation 61 to be controlled is divided into first and second-beampaths 62, 63 by a partial reflector 64. The first and second beam paths62, 63 are recombined by a second partial reflector 65 to provide theoutput beam of radiation 66. The first and second beam paths 62, 63 arerecombined at the second partial reflector 65 in such a way that theyinterfere. A reflector 67, mounted on an actuator system 68 is providedwhich, by suitable movements of the reflector 67, adjusts the length ofthe first radiation beam path 62. By adjusting the length of the firstpath length 62, the radiation from the first and second radiation beampaths 62, 63 either constructively or destructively interferes, orsomewhere between the two. Accordingly, the intensity of the output beamof radiation 66 can be controlled. A radiation beam dump 69 can beprovided to absorb the radiation rejected by the variable attenuator 60.

This arrangement allows for adjusting the mirror to provide a differencein the path lengths of the first and second radiation beam paths 62, 63to be an integer multiple of the wavelength of the radiation plus onehalf of the wavelength of the radiation. Thus, it is possible to attaina very low level of transmission of the variable attenuator to theradiation. This is because the radiation from the two radiation beampaths 62, 63 destructively interferes at the second partial reflector65. This is desirable because, as discussed above, the minimum level oftransmission attainable by the variable attenuator determines the finalaccuracy of the dose that is provided by a radiation dose control systemin which a main pulse of radiation is followed by one or more correctionpulses.

The actuator system 68 can be comprised of one or more piezoelectricactuators. Such actuators are capable of accurately and rapidlyadjusting the position of the reflector 67 over the required range ofmovement, e.g., of the order of one quarter of the wavelength of thebeam of radiation in order to adjust the path length by half thewavelength of the beam of radiation, e.g., from minimum to maximumtransmission. In a manner corresponding to the first embodimentdiscussed above, the controller 70 can include a memory 71 that storescalibration data relating the control signals provided to the actuatorsystem 68 to the level of transmission of the variable attenuator 60 tothe beam of radiation 61.

Embodiment 5

FIG. 11 depicts a variable attenuator 80 according to a fifth embodimentof the present invention. The fifth embodiment corresponds to the fourthembodiment discussed above, e.g., in which the input beam of radiation81, is divided by a first partial reflector 84 into first and secondradiation beam paths 82, 83 before being recombined by a second partialreflector 85 to provide an output beam of radiation 86. As with thefourth embodiment, the path length of the first radiation beam path 82is adjusted, for example, by means of a reflector 87, which is actuatedby an actuator system 88. The difference between the fourth and thefifth embodiments lies in the arrangement for controlling the variableattenuator.

In a similar arrangement to the third embodiment, a second beam ofradiation 91 is directed through the variable attenuator 80 andinspected by a radiation detector 92. Accordingly, a direct measurementcan be made of the level of transmission of the variable attenuator. Aswith the third embodiment, a second detector can be provided to inspectthe second beam of radiation 91 prior to it being directed through thevariable attenuator such that the input and output intensity levels canbe accurately compared.

As shown, the second beam of radiation 91 can follow the same radiationbeam path as the radiation beam 81 to be controlled. In this case,reflectors 93, 94 are provided to introduce the second beam of radiation91 to the radiation beam path of the beam of radiation 81 to becontrolled and to extract it from the path of the output beam ofradiation 86. As discussed above in relation to FIG. 3, the source ofradiation 95 for the second beam of radiation 91 can be provided withinthe variable attenuator or external to it or even external to alithographic apparatus within which the variable attenuator is used.

In any case, if a different wavelength of radiation is used for thesecond beam of radiation from that used for the beam of radiation 81 tobe controlled, the reflectors 93, 94 used to insert and extract thesecond beam of radiation 91 can be selected such that they reflectradiation of the wavelength of the second beam of radiation 91 buttransmit radiation of the wavelength of the beam of radiation to becontrolled 81. Accordingly, the second beam of radiation can beintroduced into and extracted from the beam path of the beam ofradiation to be controlled 81 without interfering with it.

In an alternative arrangement, the reflectors 93, 94 used to introduceand extract the second beam of radiation 91 from the beam path of thebeam of radiation 81 to be controlled can be polarizing beam splitters.The beam of radiation to be controlled 81 and the second beam ofradiation 91 can be orthogonally polarized at the appropriateorientations.

As a further alternative, the second beam of radiation 91 may notprecisely follow the beam path of the beam of radiation to be controlled81, but follow a parallel path (for example displaced into or out of theplane of FIG. 11). The changes of the path length for the second beam ofradiation by actuating the reflector 87 are the same as the changes inthe path length for the beam of radiation to be controlled 81.

The controller 90 is configured to generate control signals for theactuator system 88 in order to control the position of the mirror 87based on the detected intensity of the second beam of radiation 91detected by the detector 92. It will be appreciated that the radiationdetector 92 can have the same configuration as depicted in FIG. 9 b, inrelation to the third embodiment, such that the detector can detect notonly the intensity of the second beam of radiation, but also theuniformity of the intensity of the cross-section of the second beam ofradiation 91. In that case, the controller 90 can be configuredadditionally to provide control signals to the actuator system 88 inorder to adjust the position of the mirror 87 in order to optimize theintensity uniformity of the beam of radiation.

Embodiment 6

The variable attenuator of the sixth embodiment is comprised of a pairof λ/4 phase gratings. For example, phase gratings divided into aplurality of areas of a first and second type, for example having aplurality of elongate striations, and in which a phase shift induced inradiation incident on areas of the first type is a quarter of awavelength greater or less than the phase shift induced in radiationincident on the areas of the second type. As shown in FIG. 12 a, in afirst phase grating 100 this can be provided by having first area 102having a thickness t1 and second areas 103 having a smaller thicknesst2. The second grating 101 has a similar structure.

The two phase gratings are arranged parallel to one another, but atleast one is movably mounted such that the second phase grating can bemoved relative to the first phase grating between a first position,depicted in FIG. 12 a. In the first position the areas of the first type102, 104 of the first and second phase gratings 100, 101 are aligned andthe areas of the second type 103, 105 of the first and second phasegratings 100, 101 are aligned, and can be shifted to a second position,depicted in FIG. 12 b. In the second position, the areas of the firsttype 102 of the first phase grating 100 are aligned with the areas ofthe second type 105 of the second phase grating 101 and the areas of thesecond type 103 of the first phase grating 100 are aligned with theareas of the first type 104 of the second phase grating 101.Consequently, in the first position a λ/2 phase grating is effectivelycreated and in the second position no phase grating is formed at allbecause the same phase shift is introduced to all of the radiation.Accordingly, in the first position the transmission of zero orderradiation through the combined phase gratings is minimized and in thesecond position transmission of zero order radiation through thecombination of the phase gratings is maximized.

Alternatively or additionally, as illustrated in FIG. 12 c, the phasegratings 100, 101 can be separated by a distance 143 so that one or bothof the phase gratings 100, 101 lies in the “Talbot” 142 or “half-Talbot”141 plane of the other. In general, Talbot planes are defined as theplanes in which images of a planar periodic structure/grating are formedwhen the structure/grating is illuminated by a collimated beam ofradiation. Half-Talbot planes are the planes halfway in between. Thespacings between the planes can be referred to as Talbot spacings andhalf-Talbot spacings. The separation between the phase gratings 100, 101can be air (as shown), glass or a combination of the two, for example.

FIGS. 12 a-12 c show phase gratings 100, 101 arranged with the parallelrear sides of the gratings. This can represent the substrate sides ofthe grating (i.e., the non-patterned sides), facing each other and/or incontact with each other. However, the phase gratings 100, 101 couldequally be arranged so that their faces (patterned sides) face eachother or are in contact, or the face of one grating can be arranged toface the rear or substrate side of the other grating. In each case, thegratings can be in contact or separated by Talbot or half-Talbotspacings.

In one example, the phase grating 100, 101 is periodic and areas 102-105all have the same width. To facilitate manufacture, the grating periodcan be greater than or much greater than the wavelength of the incidentradiation. Also, the illumination can be collimated with a divergencethat is less than or much less than the grating diffraction angle, whichis wavelength/period of grating. It can therefore be convenient to placethe attenuator before the adjuster AD (FIG. 1), which increases theillumination divergence.

In an exemplary embodiment, the positions, s, of the Talbot andhalf-Talbot planes are given approximately by the expression s=m p2/λ,where s is measured from a reference grating, p is the gratingperiodicity, λ is the wavelength of incident radiation, and m is aninteger (1, 2, 3 . . . etc.). Odd values of m represent half-Talbotplanes/separations and even values of m represent full Talbot planes.For λ=193 nm and p=10 μm, s=m*518 μm for the case where the gratings areseparated by an air gap. To achieve good efficiency, the gratings shouldbe positioned within about 2 μm of the Talbot and half-Talbot planes inthis example. If the gap is filled with a grating substrate material,such as glass, for example, the separations will be increased inproportion to the refractive index of the filler material.

Accordingly, as depicted in FIG. 13, the variable attenuator 110 of thesixth embodiment of the present invention comprises two λ/4 phasegratings 111, 112. The two gratings 111, 112 are arranged mutuallyparallel and an actuator system 113 configured to adjust the relativeposition of the phase gratings in order to switch them between the firstand second positions discussed above in relation to FIGS. 12 a and 12 b.This can be done so that the level of transmission of the variableattenuator is controlled between a minimum level of transmission and amaximum level of transmission. The actuator system 113 can, for example,be comprised of a plurality of piezoelectric actuators which are capableof rapidly and accurately controlling the relative position of the twophase gratings 111, 112. Alternatively, the actuators can beelectromagnetic.

In order to maintain the attainable levels of contrast, a barrier 116with an aperture 117 can be provided that only permits zero orderradiation to pass as the output beam of radiation 118 from the variableattenuator.

A lens system can be added to help separate the desired zero-order beamfrom the other-order beams. Although such an arrangement can addcomplexity, it can shorten the distance between grating 112 and stops116.

A controller 114 is provided that generates the control signalsnecessary for the actuator system 113. The controller 114 can include amemory 115 that contains calibration data, relating the control signalsto be provided to the actuator system 113 to the level of transmissionof the variable attenuator 110 to the input beam of radiation 115.

Embodiment 7

FIG. 14 depicts a variable attenuator 120 according to the seventhembodiment of the present invention. The variable attenuator of theseventh embodiment corresponds to that of the sixth embodiment, e.g., iscomprised of a pair of mutually parallel phase gratings 121, 122 and anactuator system 123 that controls the relative position of the pair ofphase gratings in response to control signals provided by a controller124.

However, the seventh embodiment of the variable attenuator includes atleast one position sensor 125 that measures the actual relative positionof the first and second phase gratings 121, 122. Accordingly, thecontroller 124 can determine a desired relative position of the phasegratings 121, 122 in order to provide a desired level of transmission ofthe variable attenuator 120 to the input beam of radiation 126.Subsequently it provides the control signals to the actuator system 123necessary to minimize the difference between the desired relativeposition and the measured relative position.

The position sensor system can comprise one or more capacitive sensorsand be configured simply to measure the relative position of the phasegratings 121, 122 in the direction of relative movement of the phasegratings necessary to switch between the first position and the secondposition depicted in FIGS. 12 a and 12 b. Additionally, however, theposition sensors can monitor the separation of the phase gratings 121,122 and/or the relative angle of the phase gratings 121, 122.Accordingly, the controller 124 can provide control signals to theactuator system 123 in order to adjust the separation of the phasegratings 121, 122 and/or the relative angle of the phase gratings 121,122. This is beneficial because in order for the maximum and minimumlevels of transmission to be provided for any of the variableattenuators of the invention that comprise two phase gratings, the phasegratings 121, 122 should be maintained as close to parallel as possibleand should be very closely spaced or separated by specific distances,approximated by so-called Talbot planes or by half Talbot planes. In thelatter case, the relative positions of the phase gratings 121, 122required for maximum and minimum levels of transmission are exchanged.

Embodiment 8

FIG. 15 depicts an eighth embodiment of a variable attenuator 130according to the present invention. As depicted, the eighth embodimentcorresponds to the sixth and seventh embodiments of the presentinvention, comprising two phase gratings 131, 132, an actuator system133 for adjusting the relative position of the phase gratings 131, 132,and a controller 135 that provides control signals to the actuatorsystem 133.

In the eighth embodiment, a second beam of radiation 136 is directedthrough the phase gratings 131, 132 and inspected by a radiationdetector 137. As with the previous embodiments involving a second beamof radiation, the second beam of radiation 136 can be of a differentwavelength to the beam of radiation 138 to be controlled by the variableattenuator 130. Likewise, the source 139 for the second beam ofradiation 136 can be arranged within the variable attenuator 130 or canbe external to the variable attenuator or external to a lithographicapparatus within which the variable attenuator is used. The position ofthe source 139 and detector 137 can be exchanged.

The second beam of radiation 136 passes through the phase gratings 131,132 at a separate location away from the beam of radiation 138 to becontrolled. Accordingly, the second beam of radiation does not interferewith the beam of radiation to be controlled 138. The inspection of thesecond beam of radiation that is passed through the phase gratings 131,132 provides a direct measurement of the level of transmission of thevariable attenuator for that beam of radiation. As with previousembodiments, a second detector can be provided to inspect the radiationbefore it passes through the phase gratings 131, 132 to provide anaccurate comparison of the intensity of the radiation before and afterpassing through the variable attenuator.

The controller 135 can use the data from the radiation detector 137 inorder to control the actuator system 133 in order to provide therequired level of transmission of the variable attenuator 130 to be beamof radiation 138 to be controlled. If the radiation of the second beamof radiation 136 is different from that of the beam of radiation to becontrolled 138, then the level of transmission of the variableattenuator 130 to the second beam of radiation 136 may not directlyreflect the level of transmission of the variable attenuator 130 to thebeam of radiation 138 to be controlled. However, the controller 135 canbe provided with a calculation module that can calculate the level oftransmission of the variable attenuator 130 to the beam of radiation 138to be controlled from the level of transmission of the variableattenuator 130 to the second beam of radiation 136. Alternatively, thecontroller can include a memory that contains calibration data thatrelates the level of transmission of the variable attenuator 130 to thesecond beam of radiation 136 to the level of transmission of thevariable attenuator 130 to the beam of radiation 138 to be controlled.

As depicted in FIG. 15, the variable attenuator 130 can comprise aplurality of second beams of radiation arranged to pass through thephase gratings 131, 132 at different locations and correspondingdetectors. By comparing the transmission levels of the variableattenuator to the second beams of radiation at different locations, thecontroller can determine the adjustments needed to the separation and/orrelative angle of the phase gratings 131, 132, and provide the requisitecontrol signals to the actuator system 133.

To achieve increased dynamic range and/or resolution any one or more ofthe above described attenuators can be arranged in series (so that theradiation to be attenuated passes through the attenuators one afteranother), either in the context of a lithographic apparatus or in a moregeneral context.

Although specific reference can be made in this text to the use oflithographic apparatus in the manufacture of a specific device (e.g., anintegrated circuit or a flat panel display), it should be understoodthat the lithographic apparatus described herein can have otherapplications. Applications include, but are not limited to, themanufacture of integrated circuits, integrated optical systems, guidanceand detection patterns for magnetic domain memories, flat-paneldisplays, liquid-crystal displays (LCDs), thin-film magnetic heads,micro-electromechanical devices (MEMS), light emitting diodes (LEDs),etc. Also, for instance in a flat panel display, the present apparatuscan be used to assist in the creation of a variety of layers, e.g., athin film transistor layer and/or a color filter layer.

Although specific reference is made above to the use of embodiments ofthe invention in the context of optical lithography, it will beappreciated that the invention can be used in other applications, forexample imprint lithography, where the context allows, and is notlimited to optical lithography. In imprint lithography a topography in apatterning device defines the pattern created on a substrate. Thetopography of the patterning device can be pressed into a layer ofresist supplied to the substrate whereupon the resist is cured byapplying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

CONCLUSION

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to persons skilledin the relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the invention.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

It is to be appreciated that the Detailed Description section, and notthe Summary and Abstract sections, is intended to be used to interpretthe claims. The Summary and Abstract sections can set forth one or more,but not all exemplary embodiments of the present invention ascontemplated by the inventor(s), and thus, are not intended to limit thepresent invention and the appended claims in any way.

1. A variable attenuator configured to adjust its level of transmissionof an input beam of radiation in response to an input control signalthat represents a desired level of transmission of the variableattenuator to the beam of radiation, comprising: first and secondsemi-transparent reflectors, arranged substantially mutually paralleland such that the beam of radiation successively passes through thefirst and second semi-transparent reflectors; and an actuator systemthat is configured to control the separation of the first and secondsemi-transparent reflectors in response to the input control signal. 2.The variable attenuator of claim 1, wherein the beam of radiation isincident on the first semi-transparent reflector at an oblique angle. 3.The variable attenuator of claim 1, further comprising a controller thatis configured to provide control signals to the actuator system in orderto control the actuator system in response to the input control signal.4. The variable attenuator of claim 3, wherein: the controller includesa memory configured to store calibration data corresponding to arelationship between the level of transmission of the attenuator and thecontrol signals; and the controller is configured to generate thecontrol signals for the actuator system in response to the input controlsignal and based on the calibration data.
 5. The variable attenuator ofclaim 3, further comprising: a position sensor system configured tomeasure the relative position of the first and second semi-transparentreflectors; and wherein the controller is configured to generate thecontrol signals for the actuator system in response to the input controlsignal and based on the measured relative position of the first andsecond semi-transparent reflectors.
 6. The variable attenuator of claim5, wherein: the controller is configured to determine a desired relativeposition of the first and second semi-transparent reflectors based onthe input control signal and to generate the control signals for theactuator system based on the difference between the desired and measuredrelative position of the first and second semi-transparent reflectors.7. The variable attenuator of claim 5, wherein: the actuator system isfurther configured to control the relative angular position of the firstand second semi-transparent reflectors; and the controller is configuredsuch that it provides control signals to the actuator system to controlthe relative angular position of the first and second semi-transparentreflectors based on the measured relative position of the first andsecond semi-transparent reflectors.
 8. The variable attenuator of claim3, further comprising: a source of radiation configured to generate asecond beam of radiation that is incident on one of the first and secondsemi-transparent reflectors at a different angle from that at which thefirst beam of radiation is incident on the first semi-transparentreflector; and a detector system that is configured to inspect thesecond beam of radiation after it has passed through the first andsecond semi-transparent reflectors, wherein the detector system isconfigured to measure the intensity of the second beam of radiationpassing through the variable attenuator, and wherein the controller isconfigured to generate the control signals for the actuator system inresponse to the input control signal based on the measured intensity ofthe second beam of radiation.
 9. The variable attenuator of claim 8,wherein: the detector system is further configured to measure theuniformity of the radiation intensity across a cross section of thesecond beam of radiation; the actuator system is further configured tocontrol the relative angular position of the first and secondsemi-transparent reflectors; and the controller is configured such thatit provides control signals to the actuator system to control therelative angular position of the first and second semi-transparentreflectors based on the measured uniformity of the radiation intensityacross the cross section of the second beam of radiation.
 10. A variableattenuator configured to adjust its level of transmission to an inputbeam of radiation in response to an input control signal that representsa desired level of transmission of the variable attenuator to the beamof radiation, comprising: a radiation beam splitter that divides thebeam of radiation onto first and second radiation beam paths; aradiation beam combiner that re-combines radiation from the first andsecond radiation beam paths, such the re-combined radiation interferesand produces an output beam of radiation; and a radiation beampathlength controller configured to control a pathlength of the firstradiation beam path in response to the input control signal in order tocontrol the interference of the radiation from the first and secondradiation beam paths.
 11. The variable attenuator of claim 10, whereinthe radiation beam pathlength controller comprises a reflector and anactuator system configured to control the position of the reflector,such that the radiation beam pathlength controller can alter thepathlength of the first radiation beam path.
 12. The variable attenuatorof claim 11, further comprising a controller that is configured toprovide control signals to the actuator system in order to control theactuator system in response to the input control signal.
 13. Thevariable attenuator of claim 12, wherein: the controller includes amemory configured to store calibration data corresponding to arelationship between the level of transmission of the attenuator and thecontrol signals; and the controller is configured to generate thecontrol signals for the actuator system in response to the input controlsignal based on the calibration data.
 14. The variable attenuator ofclaim 12, further comprising: a source of radiation configured togenerate a second beam of radiation and to direct it to the radiationbeam splitter, such that the second beam of radiation is divided intothe first and second beam paths and re-combined by the radiation beamcombiner; and a detector system that is configured to inspect the secondbeam of radiation once it has been re-combined by the radiation beamcombiner; wherein the detector system is configured to measure theintensity of the second beam of radiation, and wherein the controller isconfigured to generate the control signals for the actuator system inresponse to the input control signal based on the measured intensity ofthe second beam of radiation.
 15. The variable attenuator of claim 14,wherein: the detector system is further configured to measure theuniformity of the radiation intensity across the cross section of thesecond beam of radiation; the actuator system is further configured suchthat it can control the angle of the reflector of the radiation beampathlength controller relative to a beam of radiation propagatingthrough the first radiation beam path which, in turn, controls therelative angle of incidence of the radiation from the first and secondradiation beam paths in the radiation beam combiner; and the controlleris configured such that it provides control signals to the actuatorsystem to control the angle of the reflector of the radiation beampathlength controller based on the measured uniformity of the radiationintensity across the cross section of the second beam of radiation. 16.A variable attenuator configured to adjust its level of transmission toan input beam of radiation in response to an input control signal thatrepresents a desired level of transmission of the variable attenuator tothe beam of radiation, comprising: first and second phase gratings; andan actuator system; wherein the first and second phase gratings arearranged substantially mutually parallel and so that the beam ofradiation is initially incident on the first phase grating and then isincident on the second phase grating; wherein each of the phase gratingscomprises a plurality of regions of a first type and a plurality ofregions of a second type, wherein the phase gratings are constructedsuch that, for each phase grating, a phase shift introduced to the beamof radiation passing through the regions of the first type is a quarterof a wavelength of the beam of radiation input to the variableattenuator greater than for the regions of the second type, and whereinactuator system is configured to adjust the relative positions of thefirst and second phase gratings in response to the input control signalat least between a first position, in which radiation passing throughregions of the first and second type of the first phase gratingsubsequently passes through regions of the first and second type,respectively, of the second phase grating, and a second position, inwhich radiation passing through regions of the first and second type ofthe first grating subsequently passes through regions of the second andfirst type, respectively, of the second phase grating.
 17. The variableattenuator of claim 16, wherein: zero order radiation that has passedthrough the first and second phase gratings is directed as an output ofthe variable attenuator; and the variable attenuator comprises at leastone radiation dump to which higher positive or higher negative orders,or both, of radiation that has passed through the first and second phasegratings are directed.
 18. The variable attenuator of claim 16, furthercomprising a controller that is configured to provide control signals tothe actuator system in order to control the actuator system in responseto the input control signal.
 19. The variable attenuator of claim 18,wherein: the controller includes a memory configured to storecalibration data corresponding to the relationship between thetransmission level of the attenuator and the control signals; and thecontroller is configured to generate the control signals for theactuator system in response to the input control signal and based on thecalibration data.
 20. The variable attenuator of claim 18, furthercomprising: a position sensor system configured to measure the relativeposition of the first and second phase gratings; and wherein thecontroller is configured to generate the control signals for theactuator system in response to the input control signal and based on themeasured relative position of the first and second phase gratings. 21.The variable attenuator of claim 20, wherein the controller isconfigured to determine a desired relative position of the first andsecond phase gratings based on the input control signal and to generatethe control signals for the actuator system based on the differencebetween the desired and measured relative position of the first andsecond phase gratings.
 22. The variable attenuator of claim 20, wherein:the actuator system is further configured to control a relative angularposition of the first and second phase gratings; and the controller isconfigured such that it provides control signals to the actuator systemto control the relative angular position of the first and second phasegratings based on the measured relative position of the first and secondphase gratings.
 23. The variable attenuator of claim 18, furthercomprising: a source of radiation configured to generate a second beamof radiation that is directed to pass through the first and second phasegratings at a location different from the first beam of radiation; and aradiation detector configured to detect intensity of zero orderradiation derived from the second beam of radiation that has passedthrough the first and second phase gratings, wherein the controller isconfigured to generate the control signals for the actuator system inresponse to the input control signal based on the intensity measured bythe radiation detector.
 24. The variable attenuator of claim 23,wherein: the source of radiation and the radiation detector form a firstsensor for providing information to the controller related to therelative position of the first and second phase gratings; the variableattenuator comprises at least one further sensor corresponding to thefirst sensor generating a corresponding beam of radiation that passesthrough the first and second phase gratings at a location different fromthe first and second beams of radiation; and the controller isconfigured to generate the control signals for the actuator system inresponse to the input control signal based on the information providedby the sensors.
 25. The variable attenuator of claim 16, wherein thesecond phase grating is separated from the first phase grating and liesin a Talbot plane of the first grating or a half-Talbot plane of thefirst grating.
 26. A lithographic apparatus, comprising: an illuminationsystem configured to condition a pulsed beam of radiation; a variableattenuator configured such that it attenuates the intensity of at leastone pulse of the pulsed beam of radiation, wherein the variableattenuator comprises one of, a) two semi-transparent reflectors and apath length controller, b) a beam splitter, a beam combiner, and a pathlength controller, or c) two phase gratings and a path lengthcontroller, and a control system, configured to determine a desiredintensity of a pulse of radiation and to provide a control signal to thevariable attenuator, corresponding to a desired level of transmission ofthe variable attenuator to the beam of radiation, necessary to attenuatethe pulse to the desired intensity.
 27. The lithographic apparatus ofclaim 26, comprising a plurality of the variable attenuators arranged inseries.
 28. A device manufacturing method, comprising: forming avariable attenuator using first and second semi-transparent reflectorsthat are arranged substantially mutually parallel, such that a pulsedbeam of radiation successively passes through the first and secondsemi-transparent reflectors; controlling separation of the first andsecond semi-transparent reflectors in response to an input controlsignal, attenuating an intensity of at least one pulse of the pulsedbeam of radiation using the variable attenuator, which is configured toadjust its level of transmission to an input beam of radiation inresponse to the input control signal, which represents a desired levelof transmission of the variable attenuator to the beam of radiation;modulating a pulsed beam of radiation; and projecting the modulated beamonto a substrate.
 29. A device manufacturing method, comprising: forminga variable attenuator using a radiation beam splitter that divides apulsed beam of radiation into first and second radiation beam paths anda radiation beam combiner that re-combines radiation from the first andsecond radiation beam paths such that it interferes and produces anoutput beam of radiation; using a radiation beam pathlength controllerto control the pathlength of the first radiation beam path in responseto an input control signal in order to control the interference of theradiation from the first and second radiation beam paths; attenuatingintensity of at least one pulse of the pulsed beams of radiation usingthe variable attenuator that is configured to adjust its level oftransmission to an input beam of radiation in response to the inputcontrol signal, which represents a desired level of transmission of thevariable attenuator to the beam of radiation; modulating the pulsed beamof radiation from the variable attenuator; and projecting it onto asubstrate.
 30. A device manufacturing method, comprising: forming avariable attenuator from first and second phase gratings that arearranged substantially mutually parallel, such that a pulsed beam ofradiation is initially incident on the first phase grating and, havingpassed through the first phase grating, is incident on the second phasegrating; forming a plurality of regions of a first type and a pluralityof regions of a second type on each of the phase gratings; constructingthe phase gratings such that, for each phase grating, a phase shiftintroduced to the pulsed beam of radiation passing through the regionsof the first type is a quarter of the wavelength of the beam ofradiation input to the variable attenuator greater than for the regionsof the second type; adjusting the relative positions of the first andsecond phase gratings in response to an input control signal at leastbetween a first position, in which the radiation passing through regionsof the first and second type of the first phase grating subsequentlypasses through regions of the first and second type, respectively, ofthe second phase grating, and a second position, in which radiationpassing through regions of the first and second type of the firstgrating subsequently passes through regions of the second and firsttype, respectively, of the second phase grating; attenuating intensityof at least one pulse of the pulsed beam of radiation using the variableattenuator configured to adjust its level of transmission to an inputbeam of radiation in response to the input control signal, whichrepresents a desired level of transmission of the variable attenuator tothe beam of radiation, comprising; and modulating the pulsed beam ofradiation; and projecting the modulated beam onto a substrate.